Figure 2. Tree Mortality from Drought and Wildfire. Source: California's Tree Mortality Task Force (http://www.fire.ca.gov/treetaskforce/)

Trees in California's Sierra Nevada region have recently been experiencing unprecedented levels of mortality. This summer, the US Forest Service found an additional 26 million dead trees in California since October 2015 (Figure 2). However, tree mortality has been on the rise in the Sierra Nevada region with a total loss of 66 million trees since 2010 [1].

The US Forest Service has attributed a combination of the on-going drought, rise in bark beetle infestation, and rising temperatures for exacerbating the rate of tree mortality. This combination of stressors has not only caused additional concern for increased wildfire risk, but also has raised environmental concern for the health of our forest ecosystems. Although tree mortality is generally a clear signal that forest ecosystems are in danger, it is often too late to intervene and improve the health of the ecosystem so that tree mortality can be avoided. Warranting a need for early warning indicators.

The Impact of Groundwater Depletion on Our Forests

California's Sierra Nevada isn't the only place in the state where forest ecosystems are at risk of collapse due to drought and increased temperatures - our riparian forests are in trouble too. Riparian forests are found near rivers and streams, and often depend on a combination of surface water and groundwater to meet their water needs. When groundwater is required to support all or some of the water needs of a riparian forest, it is also referred to as a Groundwater Dependent Ecosystem. Part of the reason why this news has gone under the radar is because Groundwater Dependent Ecosystems are poorly understood, and hence, monitoring or management of Groundwater Dependent Ecosystems in California has been lacking.

Groundwater is particularly important for these forests in California's dry summers and drought periods when less surface water is available. Although riparian forests, like many ecosystems, have evolved complex physiological and biochemical adaptations to adjust and adapt to short-term water stress, if water stress is prolonged these coping strategies become inadequate and result in a progressive decline in their condition or status. Some ecological impacts to long-term water stress may be recoverable with or without management efforts, but others may be irreversible and result in a loss of ecosystem habitat or to an important species. Ecological response functions to changes in groundwater availability can impact only one individual species or multiple species within an ecosystem. Decreasing groundwater availability progressively impact biological functions such as growth, reproduction, recruitment, mortality, and ecosystem structure and function (Figure 3).

Figure 3. Ecological Responses to Groundwater Depletion [2].

A Need for Early Warning Indicators

The incorporation of indicators (Table 1) into monitoring programs that can serve as an early-warning sign to resource managers would enhance opportunities for intervention and protect the health of our ecosystems. Choosing appropriate indicators will vary between ecosystems depending on which plants and animals are present (ecosystem structure), as well as their interdependencies (ecosystem function). However, the incorporation of early-warning indicators into monitoring programs can provide useful data to inform policy and management actions necessary to keep our forests healthy.

Table 1. Potential indicators for evaluating the health of an ecosystem.

References:

1. USDA (2016). Forest Service Survey Finds Record 66 Million Dead Trees in Southern Sierra Nevada. News Release. Available at: http://www.fs.fed.us/news/releases/forest-service-survey-finds-record-66-million-dead-trees-southern-sierra-nevada

2. Rohde, M.M., R. Froend, J. Howard (2017). A Global Synthesis of Managing Groundwater Dependent Ecosystems Under Sustainable Groundwater Policy. Groundwater, 55(3): pp.293-301. Available at: https://onlinelibrary.wiley.com/doi/full/10.1111/gwat.12511